Emperion Noncemented Modular Femoral Component – Can Modularity Solve Any Primary Total Hip Replacement Problem?

Abstract

The Emperion femoral component (Smith & Nephew, Memphis, Tenn) is a modern incarnation of the S-ROM femoral component (Depuy, Warsaw, Ind). This fit-and-fill modular design has had great clinical success for more than 20 years. The Emperion can be used to address almost all clinical scenarios in primary total hip replacement (THR). The modularity can uniquely weather difficult proximal femoral deformities encountered during THR, including developmental dysplasia of the hip and slipped capital femoral epiphysis. In addition, the nomenclature attached to this implant has made using the device and its instruments user friendly.

The Emperion femoral component (Smith & Nephew, Memphis, Tenn) is a modern design of a modular stem for primary total hip replacement (THR) (Figure 1). The design is derived from the S-ROM femoral component (Depuy, Warsaw, Ind). It is a two-piece component comprising a proximal sleeve and stem. The construct is a proximal-bearing, proximal-fixation implant. Porous coating on the sleeve provides ultimate fixation. The porous surface of the Emperion is hydroxyapatite coated. The sleeve and stem are implanted independently. The stem is passed through and locked to the sleeve using a morse taper. The orientation of the sleeve and stem are independent, allowing the implant to address deformity while optimizing anteversion of the entire reconstruction. The femoral shaft and the proximal femoral metaphysis are machined independently to maximize “fit and fill.”

Figure 1: Emperion noncemented modular femoral component.

The concept of “fit and fill” is significantly linked to this design. Fit is achieved when contact is made with points on opposite cortices within the femoral canal or metaphysis as projected on the anteroposterior and lateral radiographs. In other words, a well-fitting implant has at least 1 point of contact medially and laterally and anteriorly and posteriorly. When trying to deal with separate anatomies (proximal femoral metaphysis and femoral shaft), a monolithic stem may fall short addressing both concurrently. Fit is clearly maximized using the modularity of Emperion or S-ROM.

Fill is defined as matching the endosteal contours of the metaphysis and shaft. The modularity of the design allows for mixing and matching shapes of potential implants to optimize fill.

The S-ROM design has been available for over 20 years. A number of reports have been published on the effectiveness of this design. Cameron¹ reported intermediate 3- to 6-year follow-up on 45 primary and 3 revision hips. At last follow-up, 98% of cases had good to excellent results. There were no mechanical failures at the modular junctions. No revisions were reported. Christie et al²reported 4- to 7-year follow-up on 175 hips in 159 patients. This was a retrospective multicentered study of primary THR procedures. Harris Hip scores improved from an average of 35 points preoperatively to an average of 91 points postoperatively. There was a 6% femoral lysis rate in this patient group, but no lysis was found at the modular junction. This implies that there was no particle production at the modular junction. Only 1 stem was revised for aseptic loosening, and 12 patients (7%) complained of thigh pain. Sporer et al³reported on 135 hips in 122 patients with an average follow-up of 5 years. This retrospective study found that 93% of patients had little or no pain. Of these patients, 95% were very or extremely satisfied with the surgical results. No revisions were reported. There was a 7% lysis rate, but no lysis was found at the modular junction. Two patients had thigh pain. Tanzer et al⁴ reported retrospectively on 59 primary THRs with an average follow-up of 8.4 years. Harris Hip scores improved from an average of 40 points to an average of 89 points. Of the patients, 88% had good to excellent results at last follow-up. Overall pain was absent or slight in 95%. Thigh pain was reported in 10%. No stem revisions were reported. Ten percent showed bony lysis at the proximal sleeve.

Lysis is a theoretic concern at metal junctions because of the potential for particle production. Several studies have been done^5-7indicating a paucity of metal debris production at the sleeve/stem junction in the S-ROM design. In addition, there is debate over whether modularity is needed at all in primary THR.⁸ With the overwhelming success and ease of use of nonmodular designs, it may not be necessary to expose a patient to added cost, the pitfalls of modular interfaces, and technical difficulties presented by a modular design.

Figure 2: Dorr classification for proximal femoral anatomy

Figure 2: Dorr classification for proximal femoral anatomy. Left, Dorr type A femur; center, Dorr type B femur; and right, Dorr type C femur.

Can This Design Solve All Clinical Scenarios in Primary THR?

In general, the Emperion design can solve most, if not all, clinical scenarios faced in primary THRs. Using the Dorr classification⁹ (Figure 2), Dorr type A and B femurs are probably most amenable to the use of this design (Figure 3, A and B). A Dorr type C femur (stovepipe, osteopenic proximal femur) may present more of a challenge proximally because of the lack of contour; however, the various sizes and shapes available in the proximal sleeves make the arthroplasty possible.


Figure 3: A, AP radiograph Dorr B proximal femur. B, AP radiograph THA with Emperion femoral component in Dorr B femur.

The greatest strength of this design is its ability to handle the most difficult variations of anatomy. Clinical conditions such as severe anteversion of the femoral neck as in developmental dysplasia of the hip (DDH), severe retroversion of the femoral neck as in slipped capital femoral epiphysis, fracture malunion, and proximal femoral osteotomy can be addressed successfully with this design.

The treatment of DDH is an excellent example of the strength of this design (Figure 4, A and B). Typically, the femoral neck is severely anteverted. The femoral neck is also hypoplastic, making for small hip offset. The intermedullary canal of the femoral shaft is frequently very narrow, approaching a diameter of 9 mm. Often, there is overgrowth of the greater trochanter. This multifaceted deformity is clearly difficult to treat with a monolithic stem.


Figure 4: A, AP radiograph hip with DDH. B, AP radiograph THA for DDH using the SROM fem-oral component.

For each Emperion stem, there are four proximal sleeve sizes each with three spout sizes allowing the femoral shaft and the proximal metaphysis to be addressed separately (Figure 5). The ability to freely rotate the stem 360° relative to the proximal sleeve allows for the reconstruction of normal anteversion regardless of the position of the proximal sleeve (Figure 6). Therefore, the proximal sleeve can be positioned for best fit even in severe anteversion without compromising the ability to restore the normal mechanics of the joint. This cannot be done with a monolithic stem. Multiple neck options allow for reconstitution of normal offset and center of rotation.

Figure 5: Sleeve offerings per stem

Figure 5: Sleeve offerings per stem; 3 spout sizes per sleeve size.

Figure 6: The stem can spin 360° independent from the sleeve

Figure 6: The stem can spin 360° independent from the sleeve.

Advantages of the Emperion Design

The surgical techniques for the Emperion and the S-ROM are similar. The femoral shaft is reamed first. The proximal metaphysis is reamed next. Finally, the calcar is prepared using a milling device.

Preparing the femoral shaft determines the size of the stem. For the Emperion, there are 4 sleeve sizes for each stem diameter. Each sleeve has 3 spout sizes. Each stem has standard offset and high offset options. The 9-mm stem also has a reduced offset option.

One major advantage of the Emperion is its ease of use. The nomenclature of the instrumentation and implants are easier to understand compared with that of the S-ROM. The proximal sleeve name carries the stem size with which it is associated. Therefore, an 11-mm sleeve is used with the 11-mm stem. As indicated above, there are 4 sleeve sizes for each stem (small, medium, large, and extra large). There are 3 spout sizes (1, 2, and 3). This is a major departure from the S-ROM nomenclature. The instruments and trial implants are also color coded. These improvements make for an easy workflow. The packaging of the final implants is also color coded, ensuring the acquisition of the appropriate materials.

Other advantages of the Emperion include a circulotrapezoidal neck that reduces impingement and a polished bullet tip stem that reduces endosteal abutment and reduces thigh pain.

Conclusion

The Emperion modular femoral component can address most cases seen in primary THR. The modular S-ROM-like design has had clinical success for more than 20 years. Difficult anatomic variations such as DDH are adequately reconstructed using this design. Moreover, the Emperion system is significantly more user-friendly than its counterpart, the S-ROM system. More clinical experience with this device is needed to determine conclusively that it will perform equally well as the S-ROM implant.

References

Cameron HU. The 3-6 year results of a modular noncemented low-bending stiffness hip implant. J Arthroplasty. 1993;8:239-243.
Christie MJ, DeBoer DK, Trick LW, et al. Primary total hip arthroplasty with use of modular SROM prosthesis. J Bone Joint Surg. 1999;81A:1707-1716.
Sporer SM, Obar RJ, Bernini PM. Primary total hip arthroplasty using a modulary proximally coated prosthesis in patients older than 70: two to eight year results. J Arthroplasty. 2004;19(2):197-203.
Tanzer M, Chan S, Brooks CE, et al. Primary cementless total hip arthroplasty using a modular femoral component: a minimum 6 year follow-up. J Arthroplasty. 2001;16:64-70.
Bobyn JD, Dujovne AR, Krygier JJ, et al. Surface analysis of the taper junctions of retrieved and in vitro tested modular hip prostheses. In: Biological, Materials, and Mechanical Considerations of Joint Replacement: Current Concepts and Future Directions. New York: Raven Press; 1993:287-301
Bobyn JD, Tanzer M, Krygier JJ, et al. Concerns with modularity in total hip arthroplasty. Clin Orthop Relat Res. 1994;298:27-36.
Krygier JJ, Dujovne AR, Bobyn JD, et al. Fatigue behavior of a titanium femoral hip prosthesis with proximal sleeve-stem modularity. J Appl Biomater. 1994;5:195-201.
Lombardi AV, Mallory TH, Fada RA, et al. Stem modularity: rarely necessary in primary total hip arthroplasty. Orthopaedics. 2002;25:1385-1387.
Dorr LD. Structural and cellular assessment of bone quality of proximal femur. Bone. 1993;14:231-242.

Author

From the Department of Orthopaedic Surgery, Long Island Jewish Medical Center, North Shore – LIJ Healthcare System, Great Neck, NY 11021.

Dr Marwin is a consultant for Smith & Nephew Orthopaedics.

Correspondence should be addressed to Scott Marwin, MD, FAAOS, Department of Orthopaedic Surgery, Long Island Jewish Medical Center, North Shore, 900 Northern Blvd, Suite 100, Great Neck, NY 11021.